After initially¹ not seeing any RPN calculators for 9front, I decided to make my own that runs in a GUI. It has basic +-\×/, as well as some trigonometric functions and exponentiation. No calculator tape for now. You can also snarf and paste from / to the calculator. The source is at https://codeberg.org/ccf/rpn9

¹I have just seen a text-based RPN calculator, https://shithub.us/flowerss/rpn/HEAD/info.html, of similar functionality